It's Not Summer Without You, Jenny Han

Isobel Conklin has spent pretty much every summer of her life at the beach house that belongs to her mother's best friend Susannah with Susannah and her two boys. But when Susannah gets cancer everything changes. Belly, as she's called by the boys, has to come to terms with the changes in circumstances and in her feelings toward the brothers. This book is a sequel to the book, The Summer I Turned Pretty. If you love a book with a little romance, a little mystery, and some tears, this would be a good choice.